The invention relates to methods for depositing materials onto substrate surfaces or into porous solids.
Thin films of materials such as metals, semiconductors, or metal oxide insulators are of great importance in the microelectronics industry. Fabrication of integrated circuits involves formation of high purity thin films, often with multiple layers, on patterned substrates. One of the most common methods for producing thin films is chemical vapor deposition (CVD). In thermal CVD, volatile precursors are vaporized under reduced pressure at temperatures below their thermal decomposition temperature and transported by means of a carrier gas into an evacuated chamber containing a substrate. The substrate is heated to high temperatures, and thermolysis at or adjacent to the heated substrate results in the surface deposition of the desired film. For a general reference on CVD see: Hitchman et al., eds., Chemical Vapor Deposition Principles and Applications (Academic Press, London, 1993).
Thin films have also been formed using supercritical fluids. For example, Murthy et al. (U.S. Pat. No. 4,737,384) describes a physical deposition method in which a metal or polymer is dissolved in a solvent under supercritical conditions and as the system is brought to sub-critical conditions the metal or polymer precipitates onto an exposed substrate as a thin film. Sievers et al. (U.S. Pat. No. 4,970,093) describes a standard CVD method in which organometallic CVD precursors are delivered to a conventional CVD reactor by dissolving the precursors in a supercritical fluid solvent. The solvent is expanded to produce a fine precursor aerosol which is injected into the CVD reactor under standard CVD conditions, i.e., pressures less than or equal to 1 atmosphere, to deposit a thin film on a substrate.
Louchev et al. (J. Crystal Growth, 155:276-285, 1995) describes the transport of a precursor to a heated substrate (700 K) in a supercritical fluid where it undergoes thermolysis to yield a thin metal (copper) film. Though the process takes place under high pressure, the temperature in the vicinity of the substrate is high enough that the density of the supercritical fluid approaches the density of a conventional gas. The film produced by this method had an atomic copper concentration of approximately 80% (i.e., 20% impurities). Bouquet et al. (Surf and Coat. Tech., 70:73-78, 1994) describes a method in which a metal oxide is deposited from a supercritical mixture of liquid and gas co-solvents at a temperature of at least 240xc2x0 C. The thin film forms as a result of thermolysis at a substrate heated to at least 290xc2x0 C.
The formation of alloys from multiple pure metal components and films containing multiple pure metal components is also of interest in microelectronic applications and device fabrication for the formation of films exhibiting, e.g., gigantic magneto resistance (GMR), increased resistance to electromigration and for modification of electrical conductivity, and for the formation of other functional layers in integrated circuits. Alloying is also used to tailor rate and selectivity for reactions over supported catalysts, improve the resistance of metal membranes to hydrogen embrittlement and to increase the hardness and corrosion resistance of barrier coatings. Mixed metal films are typically produced by physical deposition methods such as ion sputtering, which is a line-of-sight technique. In principle, CVD can also be used to produce alloy films using a combination of metal precursors. Such deposition, however, would be limited by the relative volatilities of the precursors making precise control of multi-component feed streams across the composition range difficult to achieve. Moreover, attainment of a desired composition would also depend on the relative rates of decomposition.
Thin films of palladium (Pd) and its alloys are used in technologically important applications such as catalysis, gas sensors, and H2 permselective membranes for use in gas separation and in integrated reaction/separation schemes. Moreover, Pd is a common noble metal in microelectronics, where it is used as a contact material in integrated circuits and as a seed layer for the electroless deposition of other interconnect metals. Pd films can be prepared by vacuum sputtering and electroplating. However, such techniques are generally limited to planar surfaces, limiting their applicability to applications in microelectronics where shrinking device dimensions require efficient filling of deep sub-micron, high-aspect ratio features.
High purity Pd thin films can be deposited by CVD using organopalladium compounds containing various classes of ancillary ligands as precursors. However, to maintain acceptable purity and deposition rates, temperatures usually exceed 200xc2x0 C. Moreover, because CVD is often mass-transport limited, the deposited films are expected to be non-uniform, thereby limiting efficient pore-filling and/or conformal coverage of complex surfaces. Consequently, palladium CVD has not yet been commercialized.
Copper is also used in technologically important applications, including interconnect structures in microelectronic devices. Current methods of depositing copper, such as CVD and sputtering, have not been shown to provide uniform filling of very narrow (xcx9c150 nm and less), high aspect ratio trenches or vias. As a result, copper CVD has not been practiced commercially for these applications. Other applications for copper include printed wiring boards.
The invention features new methods for depositing a material, e.g., a thin film of a pure metal, a mixed metal, or a metal alloy, or a layer, e.g., a discontinuous layer of discrete uniformly distributed clusters, onto a substrate surface or into a porous solid substrate. The methods are generally referred to herein as chemical fluid deposition (CFD). CFD involves dissolving a precursor of the material to be deposited into a solvent under supercritical or near-supcrcritical conditions and exposing the substrate (or porous solid) to the solution. A reaction reagent is then mixed into the solution and the reaction reagent initiates a chemical reaction involving the precursor, thereby depositing the material onto the substrate surface (or within the porous solid). Use of a supercritical solvent in conjunction with a reaction reagent produces high purity thin films, e.g., metal or metal alloy films, or layers of discrete high purity metal or metal alloy clusters, at temperatures that can be lower than conventional CVD temperatures. The substrate surface can include one or more layers, which may be patterned. When patterned substrates are used, e.g., having deep sub-micron, high-aspect ratio features such as trenches, CFD can provide uniform conformal coverage and uniform filling of the features.
The invention also features a two-step process that involves (1) the deposition of a catalytic seed layer, e.g., of palladium, platinum, or copper, by CFD, followed by (2) plating, e.g., electroless or electrolytic plating, or additional CFD, of more of the same metal or another metal or alloy. The seed layer need not be continuous, i.e., the seed layer can be made of clusters of deposited material, but the isolated catalytic seed clusters should be distributed uniformly in any patterns, e.g., trenches or invaginations, in the surface of the substrate. The surface can be functionalized prior to deposition using coupling agents, e.g., chlorotrimethoxysilane, or example, to control the concentration and location of the seed layer deposit.
In another two-step process, a seed layer and thin film is created simultaneously by a first thermal disproportionation step using a precursor such as copper (e.g., Cu(I)), followed by the addition of a reaction reagent such as H2 to reduce the products of the disproportionation reaction in a CFD method to obtain high yield deposition of the precursor onto a substrate.
In general, in one aspect, the invention features a method for depositing a film of a material, e.g., a metal, mixture of metals, metal alloy, metal oxide, metal sulfide, insulator; or semiconductor, onto the surface of a substrate, e.g., a silicon wafer, by i) dissolving a precursor of the material into a solvent, e.g., carbon dioxide, under supercritical or near-supercritical conditions to form a supercritical or near-supercritical solution; ii) exposing the substrate to the solution under conditions at which the precursor is stable in the solution; and iii) mixing a reaction reagent, e.g., hydrogen, into solution under conditions that initiate a chemical reaction involving the precursor, e.g., a reduction, oxidation, or hydrolysis reaction, thereby depositing the material onto the surface of the substrate, while maintaining supercritical or near-supercritical conditions.
For example, the method can be conducted so that the temperature of the substrate is maintained at no more than 200, 225, 250, 275, or 300xc2x0 C., the solvent has a reduced temperature between 0.8 and 2.0, e.g., 1.0, 1.2, 1.4, 1.6, or 1.8, the solvent has a density of at least 0.1 g/cm3, e.g., 0.125, 0.15, 0.175, or 0.2 g/cm3, the solvent has a density of at least one third of its critical density, or so that the solvent has a critical temperature of less than 150xc2x0 C. In addition, the method can be carried out so that the temperature of the substrate measured in Kelvin is less than twice the critical temperature of the solvent measured in Kelvin, or so that the temperature of the substrate measured in Kelvin divided by the average temperature of the supercritical solution measured in Kelvin is between 0.8 and 1.7. The method can also be conducted such that the average temperature of the supercritical solution is different from the temperature of the substrate.
In some embodiments, the material comprises multiple metals and the precursor comprises multiple precursors for the multiple metals. Furthermore, the material can be a homogeneous or inhomogeneous mixture of multiple metals, for example, the material can be a platinum/nickel mixture or alloy, or a copper mixture or alloy. Moreover, gradients of varying concentrations of individual metals may be created throughout a thin film.
The substrate can be a patterned substrate, such as one used in the microelectronics industry. The patterned substrate can have submicron features, which may have an aspect ratio greater than about 2, greater than about 3, or greater than about 10. The material can be deposited to conformally cover the features. In one embodiment, the substrate is a patterned silicon wafer and the material is palladium or a palladium alloy that conformally covers the patterned features. In another embodiment, the substrate is a patterned silicon wafer and the material is copper or a copper alloy that conformally covers or fills the patterned features.
In another aspect, the invention features an integrated circuit including a patterned substrate having submicron features and a film including palladium or copper conformally covering the features. The aspect ratio of the patterned features can be greater than about 2, greater than about 3, or greater than about 10.
The invention also features a method for depositing material within a microporous or nanoporous solid substrate by dissolving a precursor of the material into a solvent under supercritical or near-supercritical conditions to form a supercritical or near-supercritical solution; ii) exposing the solid substrate to the solution under conditions at which the precursor is stable in the solution; and iii) mixing a reaction reagent into the solution under conditions that initiate a chemical reaction involving the precursor, thereby depositing the material within the solid substrate, while maintaining supercritical or near-supercritical conditions. For example, this method can be conducted such that the temperature of the solid substrate is maintained at no more than 300, 275, 250, 225, 210, 200, or 190xc2x0 C.
In another aspect, the invention features a film of a material, e.g., a metal, metal mixture, metal alloy, or semiconductor, on a substrate, the coated substrate itself, and microporous or nanoporous solid substrates having such materials deposited on and within them. One embodiment is a metal or metal alloy membrane formed within a porous solid substrate. These new substrates may or may not be prepared by the new methods. In a further aspect, the invention features an integrated circuit including the new substrates, which may be prepared by the new method.
In other embodiments, the invention features a method of depositing a seed layer of a material onto a substrate by i) dissolving a precursor of the material into a solvent to form a supercritical or near-supercritical solution; ii) exposing the substrate to the solution under conditions at which the precursor is stable in the solution; and iii) mixing a reaction reagent into the solution under conditions that initiate a chemical reaction involving the precursor, wherein the material is deposited as a seed layer onto the surface of the substrate when the substrate and the reaction reagent are in contact with the solution, while maintaining supercritical or near-supercritical conditions. The method can further include a step of depositing a metal film on the seed layer, e.g., by CFD.
The invention also includes a method of depositing a material onto a substrate by i) depositing a seed layer onto the substrate; ii) dissolving a precursor of the material into a solvent to form a supercritical or near-supercritical solution; iii) exposing the substrate and seed layer to the solution under conditions at which the precursor is stable in the solution; and iv) mixing a reaction reagent into the solution under conditions that initiate a chemical reaction involving the precursor, wherein the material is deposited onto the seed layer on the surface of the substrate when the substrate and the reaction reagent are in contact with the solution, while maintaining supercritical or near-supercritical conditions.
A variation includes a method of depositing a material onto a substrate by i) dissolving a precursor of the material into a solvent to form a supercritical or near-supercritical solution; ii) depositing a seed layer from the precursor by reduction of the precursor; and iii) mixing a reaction reagent into the solution under conditions that initiate a chemical reaction involving the precursor or reduction or decomposition products of the precursor, wherein the material is deposited onto the seed layer on the surface of the substrate when the substrate and the reaction reagent are in contact with the solution, while maintaining supercritical or near-supercritical conditions.
In another embodiment, the invention features a method of depositing a material onto a substrate by i) dissolving a precursor of the material into a solvent to form a supercritical or near-supercritical solution; and ii) depositing the material by simultaneous thermal reduction (e.g., disproportionation or thermolysis) and reaction with a reaction reagent in the solution under conditions that initiate a chemical reaction involving the precursor or reduction or decomposition products of the precursor, wherein the material is deposited on the surface of the substrate when the substrate and the reaction reagent are in contact with the solution, while maintaining supercritical or near-supercritical conditions.
The invention also includes a method of depositing a material onto a substrate by i) dissolving a precursor or mixture of precursors of the material into a solvent to form a supercritical or near-supercritical solution; and ii) adding a reaction reagent in the solution under conditions that initiate a chemical reaction involving the precursor or reduction or decomposition products of the precursor, wherein the material is deposited on the surface of the substrate when the substrate and the reaction reagent are in contact with the solution, while maintaining supercritical or near-supercritical conditions.
As used herein, a xe2x80x9csupercritical solutionxe2x80x9d (or solvent) is one in which the temperature and pressure of the solution (or solvent) are greater than the respective critical temperature and pressure of the solution (or solvent). A supercritical condition for a particular solution (or solvent) refers to a condition in which the temperature and pressure are both respectively greater than the critical temperature and critical pressure of the particular solution (or solvent).
A xe2x80x9cnear-supercritical solutionxe2x80x9d (or solvent) is one in which the reduced temperature (actual temperature measured in Kelvin divided by the critical temperature of the solution (or solvent) measured in Kelvin) and reduced pressure (actual pressure divided by critical pressure of the solution (or solvent)) of the solution (or solvent) are both greater than 0.8 but the solution (or solvent) is not a supercritical solution. A near-supercritical condition for a particular solution (or solvent) refers to a condition in which the reduced temperature and reduced pressure are both respectively greater than 0.8 but the condition is not supercritical. Under ambient conditions, the solvent can be a gas or liquid. The term solvent is also meant to include a mixture of two or more different individual solvents.
The xe2x80x9caspect ratioxe2x80x9d of a feature on a patterned substrate is the ratio of the depth of the feature and the width of the feature.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
The invention includes a number of advantages, including the use of process temperatures that are much lower than conventional CVD temperatures. A reduction in process temperature is advantageous in several respects: it aids in the control of depositions, minimizes residual stress generated by thermal cycling in multi-step device fabrication that can lead to thermal-mechanical failure, minimizes diffusion and reaction of the incipient film with the substrate and/or adjacent layers, renders the deposition process compatible with thermally labile substrates such as polymers, and suppresses thermally-activated side-reactions such as thermal fragmentation of precursor ligands that can lead to film contamination. Thus, the films produced by the process are substantially free of impurities, e.g., ligand-derived impurities.
An additional advantage of the invention is that it obviates the CVD requirement of precursor volatility since the process is performed in solution. Thus, the process can be conducted at fluid phase precursor concentrations between 10 and 10,000 times or more higher than conventional gas phase processes, which mitigates mass transfer limitations and promotes conformal coverage. Furthermore, since the process is performed under supercritical or near-supercritical conditions, the diffusivity of precursors dissolved in solution is increased relative to liquid solutions, thereby enhancing transport of precursor and reaction reagent to, and decomposition products away from, the incipient film. The supercritical fluid is also a good solvent for-ligand-derived decomposition products, and thus facilitates removal of potential film impurities and increases the rate at which material forms on the substrate in cases where this rate is limited by the desorption of precursor decomposition products. In addition, since the reactants are dissolved into solution, precise control of stoichiometry is possible.
This latter consideration permits mixed metal depositions, e.g., alloys, because transport is based on solubility rather than volatility. Thus, multicomponent films can be prepared by co-reduction of appropriate organometallic compounds, and the composition of the multicomponent film can be controlled directly by stoichiometric adjustments to the fluid phase precursor concentrations.
Another advantage of the invention is that the supercritical solution is usually miscible with gas phase reaction reagents such as hydrogen. As a result, gas/liquid mass transfer limitations common to reactions in liquid solvents are eliminated, and so excess quantities of the reaction reagent can easily be used in the reaction forming the material. Thus, the techniques produce high quality metal and metal alloy deposits of precisely tailored composition in the form of thin films, conformal coatings on topologically complex surfaces, uniform deposits within high aspect ratio features, and both continuous and discreet deposits within microporous supports. Moreover, the absence of surface tension inherent to supercritical solutions ensures complete wetting of tortuous surfaces. In addition, the elimination of the volatility requirement removes constraints on precursors, and enables the use of non-fluorinated precursors that contain environmentally benign ligands.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.